Generally, a transmitter of a wireless base station (BS) may include a plurality of antennas to obtain antenna diversity, and thus enhance reliability of the signal received at the mobile terminal in the mobile communication system.
Such methods for supporting antenna diversity between the mobile terminal and the transmitter are currently classified into three types, an open loop transmit antenna diversity method, a closed loop adaptive antenna diversity method, and a transmit antenna selection diversity method.
FIG. 1 is a schematic view for explaining of an open loop transmit antenna diversity method.
The open loop transmit antenna diversity method supports a diversity gain using two transmit antennas at the transmitter of the base station in a forward link with which the transmitter transmits a signal to the mobile terminal. Also, the open loop transmit antenna diversity method uses a 2×2 Space-Time Block Code (STBC) transmit matrix based on the Alamouti scheme. The transmit matrix is given as the following Equation 1.
                    [                                                            s                1                                                                    s                2                                                                                        -                                  s                  2                  *                                                                                    s                1                *                                                    ]                            (                  Equation          ⁢                                          ⁢          1                )            
A diversity transmission method and a transmitter of a base station using the same is provided in a mobile communication system. For this purpose, the diversity transmission method for controlling power of transmit antennas when the transmitter transmits a signal to a mobile terminal using a plurality of antennas includes (a) receiving feedback information concerning the antenna having the best channel status from the mobile terminal, (b) increasing power allocation of a selected antenna according to the feedback information by a predetermined amount and decreasing power allocation of a non-selected antenna by a predetermined amount, and (c) transmitting the signal to the mobile terminal using the power allocation controlled in (b).
In Equation 1, s1 and s2 are defined as data symbols to be transmitted at time slots 1 and 2, respectively. Also, a row is defined as a data symbol respectively transmitted from a first antenna or a second antenna, and a column is defined as a data symbol transmitted for the time slots respectively. The column may use a physically independent radio resource such as a frequency (or subcarrier) or a code as well as the time slot.
For example, when channel responses for the first and second antennas at the mobile terminal are given as h1 and h2, and signals that the mobile terminal respectively receives at time 1 and time 2 are given as r1 and r2, the received signals r1 and r2 are explained as follows.r1=h1s1+h2s2 r2=−h1s2*+h2s1*
According to the open loop antenna diversity method of FIG. 1, the mobile terminal requests no feedback information so that an additional channel is not required for a reverse link (mobile-to-BS link). Also, mobility of the mobile terminal is fast so that a performance can be preserved regardless of a rapidly varying channel response. However, since an antenna weight value is not controlled according to the channel response, the open loop antenna diversity method cannot obtain array gain. Accordingly, the open loop antenna diversity method has a degraded performance at a low speed in comparison with the closed loop adaptive antenna diversity method or the transmit antenna selection diversity method.
FIG. 2 is a schematic view for explaining a closed loop adaptive antenna diversity method.
In the closed loop adaptive antenna diversity method, the transmitter of the base station supplies the antenna array gain and the diversity gain for the mobile terminal by adjusting the antenna weights according to feedback information, when the mobile terminal measures a channel status for the respective transmit antennas and transmits the antenna weight value to the BS through the feedback channel of the reverse link.
For this purpose, a transmit matrix is given as the following Equation 2.
                    [                                                            w                1                                                                    s                1                                                                                        w                2                                                                    s                2                                                    ]                            (                  Equation          ⁢                                          ⁢          2                )            
That is, when the BS transmits the signals to the mobile terminal, a complex coefficient wi(i=1, 2) multiples with the respective signals for the respective antenna. In this case, the mobile terminal must estimate the antenna weight value for maximizing a received signal-to-noise ratio (SNR).
Accordingly, in order to adjust the antenna weights for transmitting signals on the forward link, the BS needs the feedback information supplied from the mobile terminal at the reverse link. Also, the mobile terminal must periodically transmit a few to tens of bits of feedback information to transmit detailed information concerning the complex weights w1 and w2 for the respective antennas through the feedback channel of the reverse link.
As such, since the mobile terminal transmits a large amount of feedback information through the feedback channel of a restricted transmission rate, the BS cannot cope with a rapid variation of the channel status when the mobile terminal speed is high. Accordingly, the closed loop antenna diversity method has a rapidly decreased performance at a high speed
FIG. 3 is a schematic view for explaining a transmit antenna selection diversity method.
In the transmit antenna selection diversity method, so as to supply an antenna selection diversity gain, the mobile terminal measures the channel status for the respective antenna to select the most superior antenna and transmits the selection information through the feedback channel of the reverse link to the BS, and then, the BS transmits the signal through the selected antenna. For this purpose, a transmit matrix is given as the following Equation 3.
                              [                                                                      α                  1                                                                              s                  1                                                                                                      α                  2                                                                              s                  2                                                              ]                ⁢                                  ⁢                  or          ⁢                                          [                                                                                          α                    1                                    ⁢                                      s                    1                                                                                                                    α                    1                                    ⁢                                      s                    2                                                                                                                                            -                                          α                      2                                                        ⁢                                      s                    2                    *                                                                                                                    α                    2                                    ⁢                                      s                    1                    *                                                                                ]                                    (                  Equation          ⁢                                          ⁢          3                )            
In Equation 3, αi(i=1,2) is defined as an antenna selection value and is given as 0 or 1. According to the channel status, when the first antenna is selected, it is given as α1=1 and α2=0, and when the second antenna is selected it is given as α1=0 and α2=1.
Accordingly, the transmit antenna selection diversity method can reduce an amount of feedback information to be 1 bit when two transmit antennas are used. That is, the transmit antenna selection diversity method can remarkably reduce an amount of feedback information in comparison with the closed loop adaptive antenna diversity method.
However, in the transmit antenna selection method, a wrong antenna can be selected since it cannot cope with the rapid variation of the channel status when the mobility of the mobile terminal is greater than the feedback information transmission speed as in the close loop adaptive antenna diversity method. When the wrong antenna is selected, the mobile terminal may receive a signal through the antenna in which the signal is not substantially transmitted or the channel status is bad. Accordingly, the performance is degraded when the mobile speed is high or there is an error in the feedback information.
In order to solve these problems of the open-loop transmit antenna diversity method, the closed loop adaptive antenna diversity method and the transmit antenna selection diversity method used currently, Korean patent application No 2002-38274 entitled “Adaptive transmit antenna diversity method and apparatus in a mobile communication system” (2002.7.3), Korean patent application 1998-22220 entitled “Method and apparatus for controlling a power of a forward link using a transmit diversity in a mobile communication system” (1998.6.13), and Korean patent application 2002-10229 entitled “A forward channel status information transmitting/receiving method and apparatus for supporting an antenna selection diversity in a mobile communication system” (2002.2.26) and the like disclose various diversity methods. However, these prior arts have a possibility to generate very serious errors at a higher speed since they solve the problem of the diversity method at a low speed. In addition, these prior arts have a problem in that complicated calculation is performed at a mobile terminal and a large amount of feedback information is transmitted to the transmitter.
Thus, an antenna diversity method has been required to use less of the feedback information and to avoid a decrease of the performance when the mobility is increased.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.